Disposal of sodium sulfur oxide compounds poses a serious water pollution control problem. Sodium sulfur oxide compounds have long been produced as by-products from industrial processes, and will also result from the use of sodium alkali additives in air pollution control processes.
In preparing water for use as boiler feedwater, makeup water, or in cleaning up condenser water, nuclear reactor coolant water, spent fuel storage water, sump waste water, rad wastes, deborating units, and decontamination units, ambient or recirculating water supplies are decontaminated or demineralized to remove various components, including borates, calcium, sodium, silica, radioactive components, various anionic components including sulfite and sulfate ions and the like, since presence of these components cause problems in the operation of the boilers or in waste disposal. Ambient water supplies have normally been treated by a series of demineralizers or ion exchange resins. These resins are periodically backwashed or cleaned and result in a backwash containing large amounts of sodium sulfur oxide components normally assayed as sodium sulfate. For example, such demineralizer wastes can contain from 15-25,000 parts per million of sodium sulfate. The wastes also may contain other ionic components, heavy metals, radioactive components and the like which pose disposal problems. Typical heavy metals include iron, cobalt, copper, manganese, and radioactive components include radioactive potassium, 137.sub.Cs, and 131.sub.I.
Still another source of sodium sulfate is that contained in the blowdown from nuclear or fossil fuel fired power plant cooling towers. A 2,000 megawatt nuclear station employing cooling towers typically will produce cooling tower blowdown sludge or magma in the amount of approximately 3.4 tons per hour of 90% solids material which, prior to concentration, assays approximately 8,880 ppm of Na.sup.+ and SO.sub.4.sup..sup.-2 in the ratio of 1:2. The vastness of the quantity of this sludge material is evident in the fact that approximately 15 tons per megawatt are produced each year.
Far more serious is the potential for pollution from the use of sodium alkali materials, such as sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium sulfite, and the like for SO.sub.2 control in wet or dry scrubbing of SO.sub.2 from tail or flue gases from industrial plants, smelters, paper plants, glass plants, power plants and the like which burn sulfur-containing fossil fuels such as coal or oil. The magnitude of the problem can be seen from the fact that it is estimated that from 20-35 million tons of sulfur dioxide were vented to the atmosphere in the United States in 1972 from industrial plants and power plants by burning fossil fuels containing bound sulfur.
While a principal approach to SO.sub.2 control has been wet scrubbing of tail gases with lime or limestone, these processes involve several disadvantages. The energy required to pump the water through the scrubber is relatively high and the scrubbers are prone to scale formation. Normally, there must be recycle of unreacted alkaline calcium compounds and the liquid/gas ratio must be relatively high. The inverse solubility and scaling problems lead to high capital and operating costs due to complex piping, demisters, surface contact areas and the like.
Still further, the calcium scrubber sludge containing calcium sulfite and sulfate poses very expensive and serious disposal problems. Normally, it is produced at the rate of approximately one acre foot per megawatt per year. The sludge material is the "fourth" state of matter, being a thixotropic mixture of finely divided crystalline particles and water. Normally, the material contains from 40-60% water and will not settle or completely dry out at ambient conditions. Its disposal involves the cost of transporting the water; in arid areas that water is precious or unavailable. In addition, sludge cannot be piled up above ground since the pile will flow under its own weight. This has led to the use of flocculants and scrubber ponds, which are typically clay-lined ponds, or pits scooped out of clay formations. However, calcium sulfate has a tendency to break down clay and render it relatively permeable. This then permits the soluble constituents of the scrubber sludge to leach from the pond. Normally, the starting limestone contains from 10-50% magnesium carbonate which, in the scrubber SO.sub.2 removal reaction, produces highly soluble (700 g/l) magnesium sulfate (epsom salt) pollutant which can leach from the pond due to calcium sulfate-induced permeability. Current calcium sulfate/sulfite sludge disposal costs are running as high as $20/ton.
In addition, the tail gas clean-up processes also collect fly ash and heavy metals which are originally present in the coal. The heavy metals, among them beryllium, boron, cadmium, strontium, magnesium, arsenic, chromium, barium, cobalt, fluorine, mercury, manganese, nickel, tin, sellenium, vanadium, lead, radioactive elements such as .sup.226 Ra, .sup.228 Ra, .sup.228 Th, .sup.230 Th, and .sup.232 Th, are present in various quantities in the coal. These are collected in the scrubber sludge and may be leached from the ponds.
To overcome the problems of using calcium in scrubbers, and the high capital and operating costs, there have been proposed both wet and dry systems for use of sodium alkaline compounds. The sodium compounds have the advantage of increased reactivity and a lower liquid to gas ratio due to the fact that both the alkalis and end-product sodium sulfur oxide compounds are highly soluble in the scrubber water. The scrubbers employing sodium alkaline compounds are less complex primarily because no substantial attention need be paid to the problem of scaling in the scrubber.
Dry sodium processes, principally involving the use of sodium bicarbonate injected as a dry powder upstream of a baghouse have been successfully tested on full-scale sized pilot baghouse operations for removal of SO.sub.x and particulates from oil and coal-fired power plants. The resultant baghouse filter cake material is a mixture of fly ash containing residual heavy metals, and the sulfur oxide compounds, principally sodium sulfite and sodium sulfate. The dry baghouse cake is periodically removed from the baghouse hoppers.
A hybrid process involves spraying a solution of sodium bicarbonate or carbonate at a rate which permits drying after reaction and collection of fly ash and sodium sulfite/sulfate in a cyclone and/or baghouse and/or electrostatic precipitator. Mixtures of molten alkali metal carbonates are used as SO.sub.x sorbents, producing Na, K, Li sulfites/sulfates.
However, while it would be of great advantage to adopt the use of sodium alkalis, either wet or dry for SO.sub.x air pollution control in order to take advantage of the increased efficiency as compared to calcium systems, the major problem is the fact that the resultant sodium wet scrubber liquor or the dry sodium sulfur oxide-containing waste material is water soluble. The net result is that utilities and industrial users are left with a water pollution problem after having solved their air pollution emissions problem. Although the wet or dry sodium alkali SO.sub.2 emissions control processes are generally cheaper than calcium or MgO systems, the disposal of the resultant sodium sulfur oxide wastes has been a major barrier to the adoption of such systems as a solution to SO.sub.x and particulate emissions problems.
Therefore, there is a very great need for a process which will result in reducing the solubility of the SO.sub.2 emissions control wastes or the solubility of waste sodium sulfite and sulfate from various types of industrial processes. These is also great need for a process which will simultaneously dispose of two or more of the fly or bottom ash residuals from the burning of fossil fuels such as coal or oil, potentially toxic heavy metals, or radioactive elements, which are normally present as the initial components of fossil fuels such as coal or oil or are produced in the energy-generating process (nuclear reactor power plants). There is also the need for a process which will reduce the leaching or result in the fixing or partial fixing of heavy metals, radioactive elements, and sodium sulfur oxide components permitting the thus-fixed product to be cheaply disposed of, by landfill or useful as an aggregate product (where radioactivity is nil or low).